The present study investigated the effects of interleukin-3 (IL-3) on supraventricular amoeboid microglial cells (SAMC) of neonatal BALB/c and athymic mice. After four consecutive daily intraperitoneal injections of IL- 3 at the age of 1 day postnatum and perfusion at the age of 5 days, BALB/c and athymic mice showed a 20% and 37% increase, respectively, in the number of Mac-1 positive SAMC. In mice receiving seven successive injections of IL- 3 and perfused at the age of 8 days postnatum, Mac-1 labelled SAMC were increased by 14% and 19%, respectively. The increased number of SAMC could be the result of cytokine stimulation of their progenitor cells, viz., the stem cells of bone marrow or monocytes, or the result of direct stimulation of the SAMC themselves.